Apparatus and Method for Seeding a Wind Tunnel

ABSTRACT

A portable tracer fluid injection system. The system has a vapor cloud generator which vaporizes a vapor cloud generating liquid. The vapor cloud generator is uses a sub-ohm resistive heater to vaporize small volumes of the vapor cloud generating liquid, allowing for a relatively small tracer fluid injection system. This system has the benefit of being able to target specific regions of a test object within a wind tunnel and being usable for different positions within the test section of a wind tunnel.

STATEMENT OF GOVERNMENT INTEREST

The invention described and claimed herein may be manufactured and usedby or for the Government of the United States of America for allgovernment purposes without the payment of any royalty.

FIELD OF THE INVENTION

The present invention is directed to an apparatus for seeding a windtunnel, and more particularly to a portable, low wattage, remotelycontrollable apparatus for seeding a wind tunnel.

BACKGROUND OF THE INVENTION

Wind tunnels have been used for several decades to study the effects ofmoving air on objects such as aircraft, missiles, automobiles andbuildings. A typical wind tunnel has an air intake, a test section wherethe object under consideration, or a model thereof, is staged and anexhaust. The air may be recycled through the wind tunnel in a closedloop or may be replenished.

The air moves relative to the test object so that determinations andpredictions can be made about the test object's lift and dragperformance. For example, if the test object is a wing of an aircraft,measurements as to air speed, direction and pressure may be desired asthe wing is exposed to the airflow at different angles. The test objectmay be studied in a static position under steady state air flow, dynamicconditions as the test object is moved through pitch, roll and yawsequences, dynamic conditions as mass flow rate, air speed and airpressure are varied, as well as combinations of all of the foregoing.

Test data may be collected through particle image velocity (PIV)measurements, laser Doppler velocimetry, etc. But often analytical dataare insufficient to complete the study. Data collection from the sensorsmay be too slow to capture critical data points and raw numbers may notprovide a full picture of the test results.

Accordingly, wind tunnel fluid is often seeded with small particles,such as a vapor cloud, somewhat visually resembling smoke. The vaporcloud may be generated from propylene glycol (PG), vegetable glycol(VG), etc. The seeds provide a visual trace of the flow patterns aroundthe test object. Vapor cloud seeding the fluid typically relies upon awind tunnel seeder juxtaposed with the inlet of the wind tunnel. Thevapor cloud generally fills and envelopes the test object and may evenfill the entire test section of the wind tunnel.

Commercially available seed generators often operate at high pressures,up to 14 MPa, increasing both power requirements and safety concerns.But this arrangement is unsatisfactory if the wind tunnel seeder is toosmall for the test section of the wind tunnel and further unsatisfactoryif one wishes to have more detailed study of only a limited, specificportion of the test object. Furthermore, large commercially availablewind tunnel tracer fluid seeders can be difficult to adjust on the flyduring a test and may require test personnel to make adjustments in thevicinity of the wind tunnel. The present invention seeks to overcomethese problems with known tracer fluid seeders for wind tunnels.

BRIEF SUMMARY OF THE INVENTION

In one embodiment the invention comprises a tracer fluid injectionsystem. The tracer fluid injection system has a sub-ohm resistive heatedvapor cloud generator for generating vapor cloud when a vapor cloudgenerating liquid is disposed therein. The sub-ohm vapor cloud generatorhas an inlet for receiving air from an air source, whereby air enteringthe inlet may be entrained with a vapor cloud being generated by thesub-ohm vapor cloud generator; and further has an exhaust for controlledevacuation of vapor cloud from the sub-ohm vapor cloud generator to atest section of a wind tunnel.

In one particular embodiment the tracer fluid injection system mayparticularly comprise a vapor cloud generator for generating vapor cloudfrom a vapor cloud generating liquid. The vapor cloud generatorcomprises a chamber, the chamber has an air flow region and a reservoirin fluid communication therewith, the reservoir is in operablerelationship with an electric powered sub-ohm heating coil forvaporizing a vapor cloud generating liquid containable within thereservoir into tracer fluid, a wick for transferring the liquid from thereservoir to the air flow region, the air flow region has an inlet forreceiving air from an air source, whereby air entering the inlet can beentrained with a vapor cloud generated by the sub-ohm vapor cloud coil.The air flow region has an exhaust in fluid communication with the airflow region of the chamber and in fluid communication a test section ofa wind tunnel for controlled evacuation of vapor cloud from the sub-ohmgenerator to a predetermined position of the test section.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic side elevational view of a first embodiment of atracer fluid injection system according to the present invention, shownpartially in cutaway, with the direction of air flow through the windtunnel being generally indicated by the arrows.

FIG. 2 is a scale exploded view of the vapor cloud generator of thetracer fluid injection system of FIG. 1 .

FIG. 2A is a schematic top plan view of the inlet ring shown in FIG. 2 .

FIG. 3 is a schematic side elevation view of a variant embodiment of thetracer fluid injection system having a variable cross section exhaustwith a nozzle.

FIG. 4 is a schematic side elevational view of a variant embodiment of atracer fluid injection system, having an exhaust with pluraldistributaries connected to the wind tunnel, the distributaries havingoptional flexible tubing.

FIG. 5 is a schematic side elevational view of a variant embodiment ofthe invention having plural tracer fluid injection systems fed from acommon air source, each tracer fluid injection system having an exhaust,the exhausts feeding mutually different locations of the test section.

FIG. 6A is a schematic side elevational view of a variant embodiment ofthe invention having a wind tunnel with a subatmospheric test sectionand plural tracer fluid systems with exhausts feeding mutually differentlocation of the test section.

FIG. 6B is a schematic side elevational view of a variant embodiment ofthe invention having plural tracer fluid systems fed from a common airsource, each tracer fluid injection system having an exhaust, theexhausts feeding mutually different locations of the test section.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1 , the invention comprises a tracer fluid injectionsystem 15 for use with a wind tunnel 10. The system 15 comprises a vaporcloud generator 20 having an inlet 21 and an exhaust 24. The fluid flowpath through the vapor cloud generator 20 comprises, in order, an airsource 16, the inlet 21, an air transport region 22 within a chamber 23,the exhaust 24, and a test section 11 of a wind tunnel 10.

Examining the components in more detail, the air source 16 may comprisecompressed air or simply be ambient, as further described below. Ifcompressed air is selected, the air source 16 may be a tank ofcompressed air, a direct connection to an air compressor, a feed from aplant air line 32, or any suitable means for providing air at a pressuregreater than ambient and which has sufficient differential pressure topropel the air from the source 16, through the chamber 23, to the windtunnel 10 in appropriate quantities, velocities and pressures. Ambientair may be supplied through any intake to the inlet 21 of the chamber23. Except as may be specifically claimed herein, the air source 16,wind tunnel 10 and test object 17 form no part of the present invention.

Referring to FIG. 2 and FIG. 2A and examining the invention in moredetail, the vapor cloud seeding generator 20 comprises an inlet 21, achamber 23 for receiving a vapor cloud generating liquid 50, a heatingresistor for heating a vapor cloud generating liquid 50, a power sourcefor energizing the heating resistor and an exhaust 24. The chamber 23,in turn, comprises a reservoir 27 for receiving and holding the vaporcloud generating liquid 50 and an air flow region in fluid communicationwith the reservoir 27, the inlet 21 and the exhaust 24. The chamber 23of the vapor cloud generator 20 may be made of glass, stainless steel orany other suitable material which can contain the liquid 50 withoutleakage and accommodate the restive heating as discussed below.

The air from the air supply may be directed to and communicate with theinlet 21 as described above. The inlet 21 may feed the air into anannular ring 30 which circumscribes the chamber 23 of the vapor cloudgenerator 20. The annular ring 30 has an inner perimeter 30I and anouter perimeter 30P opposed thereto. The inner perimeter 30I of theannular ring 30 may have a plurality of circumferentially spaced ports30S to equalize the air feed into the chamber 23 of the vapor cloudgenerator 20. In a preferred embodiment, the inner perimeter 30I of theannular ring 30 has a slot circumscribing the chamber 23, to providegenerally balanced airflow throughout the entire air transport region 22of the chamber 23.

A wick 26 may be provided to transport liquid 50 from the reservoir 27to the air flow region 22. The wick 26 controls transport rate throughpore volume density, cross section and surface energy. The wick 26 maybe made of a nonwoven, woven material or simple aggregated fibers. Thewick 26 material may be polyolefinic, Rayon, linen or preferably cotton.A wicking rate of about 0.1 mL/sec to about 1 mL/sec, and preferablyabout 0.3 mL/sec to about 0.6 mL/sec has been found suitable. The wick26 may range from about 2 cm to about 6 cm and preferably about 3 cm toabout 5 cm. The wetted portion of the wick 26 may expose about 2 mL toabout 6 mL of the vapor cloud generating liquid 50 to the resistiveheating element 25 at any point in time.

The transport of the vapor cloud generating liquid 50 from the reservoir27 to the air flow region 22 exposes a small amount of the liquid 50 inthe air flow region 22 which may be vaporized into the tracer fluid. Byexposing only a relatively small quantity of the vapor cloud generatingliquid 50 to the vaporization process at a given moment in time, thepower for the heating requirement is reduced, and the tracer fluidinjection system 15 can be made smaller and more portable. Alternativelyand prophetically, the vapor cloud generating liquid 50 could feed theresistive heating element 25 by gravity feed if it was desired todispose the reservoir 27 above the resistive heating element 25.

A resistive heating element 25 is juxtaposed with the wick 26. Therestive heating element 25 produces heat in response to electricitysupplied from a power source. The restive heating element 25 may be madeof a wire which has high resistance to oxidation and good stability asto form, to minimize changes in shape or position throughout the life ofthe vapor cloud generator 20.

The resistive heating element 25 may comprise a wire having a diameterof about 1 mm to about 4 mm. This diameter provides a suitablecombination of formability, stability and conversion of electric energyto heat energy. The wire should be suitable for temperatures at leastabout 1300° C. The wire may be made of a ferriticiron-chromium-aluminium alloy (FeCrAl). A suitable alloy is sold by theSandvik Group under the name Kanthal® AF.

The resistive heating element 25 is juxtaposed with the wick 26,particularly the wetted portion thereof, to vaporize a desired quantityof vapor cloud generating liquid 50. This juxtaposition provides arelatively high surface area to be exposed to the wetted area of thewick 26. The resistive heating element 25 may be formed in a sinusoidalgeometry, a series of parallel longitudinally oriented wires alignedwith the wick 26, a coil internal to a hollow wick 26 or preferably acoil circumscribing the wick 26 in order to provide the necessarysurface area to vaporize the vapor cloud generating liquid 50 in thewetted area of the wick 26.

The resistive heating element 25 may have a resistance of less than 2ohms and preferably less than 1 ohm to minimize the power requirements.Particularly the resistive heating element 25 may have a resistance ofabout 0.2 ohms to about 0.5 ohms. In a particular embodiment, theresistive heating element 25 may be switched between two or morediscrete resistance values to provide flexibility in the amount oftracer fluid generated by the vapor cloud generator 20. Resistance ismeasured throughout the entire length of the wire disposed within thevapor cloud generator 20. If desired, two or more resistive heatingelements 25 may be used, provided the size and form factor arecomplementary with the wick 26, reservoir 27 and air flow region 22 ofthe vapor cloud generator 20.

The power supply may be provided from a DC battery or supplied by the ACmains. The power supply may range from at least about 5 W, 10 W, 15, W,20 W or 25 W to not more than about 200 W, 150 W, 100 W, 75 W, 60 W or30 W. In a particularly suitable low power embodiment, the power supplymay range from about 8 W to about 16 W. A DC battery power supply mayprovide at least 1600 mAhrs and at least a 10 sec firing time to theresistive heating element 25. Table 1 below shows one possible andnonlimiting relationship between the available wattage and theresistance of the resistive heating element 25.

TABLE 1 Resistance Power Embodiment in Ohms in Watts 1 0.18 60-80 2 0.2050-60 3 0.30 35-45

The vapor cloud generating liquid 50 comprises any suitable liquid 50which can be volatilized in response to heat from the resistive heatingelement 25, does not unduly vaporize when the tracer fluid injectionsystem 15 is not in use and provides a readily observable visibleindicium of the air flow relative to a test object 17 disposed in a testsection 11 of a wind tunnel 10 during testing. Suitable liquids 50include water, polyethylene glycol, vegetable glycols,Di-Ethyl-Hexyl-Sebacat (DENS) and combinations thereof. If desired, thevapor cloud generating liquid 50 may be spiked with a dye to provide acolor which contrasts with the color of the test object 17.

Referring back to FIG. 1 , the vapor from the vapor cloud generatingliquid 50 is removed from the vapor cloud generator 20, and particularlythe flow transport region 22, to the exhaust 24. The exhaust 24comprises the entire directed flow path from the outlet of the flowtransport region 22 of the chamber 23 to the point where the vapor cloud51 enters the test section 11 of the wind tunnel 10.

Additionally, the flow of tracer fluid from the vapor cloud generator 20may be controlled by a microprocessor operably connected with the powersupply of the vapor cloud generator 20. The controller 60 of themicroprocessor may be operated by a laptop or similar device operated ina room remote from the wind tunnel 10. By removing the operator from thearea near the wind tunnel 10, safety is elevated and the operator canconcentrate on test parameters.

The controller 60 is in communication with a power supply so that thesub-ohm resistive heating element 25 can vaporize the vapor cloudgenerating liquid 50 in response to energy from the power supply. Thecontroller 60 may be programmable or manually operated in response totest conditions to vary the amount of power, duration of power and pulsewidth of power supplied to the from the power supply to the sub-ohmresistive heating element 25.

One of skill will recognize the vapor cloud generator 20 portion of thefluid tracer system 15 is portable and can be used with various windtunnels 10, as desired. For example, the vapor cloud generator 20, inlet21 and exhaust 24 of the present invention may be used at a firstposition on a test section 11 of a wind tunnel 10 for a first test andat a second position, third position, etc. of the test section 11 forsubsequent tests. Further flexibility occurs when the vapor cloudgenerator 20, inlet 21 and exhaust 24 may be used with a first windtunnel 10 for a first test and even used with an entirely different windtunnels 10 for subsequent tests.

Also the fluid tracer system 15 of the present invention provides formore focus of the tracer fluid in the test section 11, to specificallytarget a region of the test object 17 under study, without blanketingthe entire test object 17 in tracer fluid and diluting or confoundingthe visual observations during testing.

The control the of the microprocessor may allow the operator to selectstart/stop times of air supply to the wind tunnel 10 intake, start stoptimes of air flow into the wind tunnel 10, air flow volume to the inlet21, air flow volume to the wind tunnel 10 intake, test duration, powerlevel to the restive heater, etc.

Referring to FIG. 3 , the exhaust 24 may comprise a variable crosssection for reduced flow resistance until the distal end of the exhaust24 is approached by the vapor cloud 51. The distal end of the exhaust 24may comprise flexible tubing 24F, as discussed below. The exhaust 24 mayhave a cross sectional area of about 2 sq. mm to about 12 sq. mm,preferably about 3 sq. mm to about 9 sq. mm and more preferably about 5sq. mm to about 8 sq. mm. If desired, the exhaust 24 may taper to anozzle 24N for more precise placement within the test section 11 of thewind tunnel 10. The embodiment provides the benefit of decoupling lowflow resistance between from vapor cloud generator 20 to the wind tunnel10 and precise placement of the nozzle 24N at the test section 11.

Referring to FIG. 4 , an alternative embodiment of the fluid tracersystem 15 according to the present invention is shown. This embodimenthas a plurality of effective exhausts 24 from the vapor cloud generator20 to the test section 11, with three exhausts 24 being shown in anonlimiting embodiment. More particularly, this embodiment has anexhaust 24 with a single manifold 24M from the vapor cloud generator 20.A plurality of two, three, nor more distributaries 24D are joined to themanifold 24M at respective proximal ends and communicate the vapor cloud51 therethrough to the test section 11. The distal ends of thedistributaries 24D are joined to the test section 11 of the wind tunnel10 so that the vapor cloud 51 can be communicated from the vapor cloudgenerator 20 to the test object 17.

The exhaust 24 may comprise flexible tubing 24F, for convenience,portability and ease of placing the vapor cloud generator 20 portion ofthe fluid tracer injection system 15 in various positions according tothe test needs and the particular wind tunnel 10 being used. Theflexible tubing 24F may preferably extend from the exhaust 24 of thechamber 23 to the test section 11 of the wind tunnel 10. The exhaust 24may comprise TYGON® tubing 24F available from Tour Saint-Gobain.

This embodiment retains the portability, safety and flexibility benefitsof the invention. This embodiment also has the benefit of providing afocused tracer fluid to two, three or more spaced apart positions of thetest section 11 (which may be difficult to access with rigid tubing) andto particular positions of the test object 17.

Referring to FIG. 5 , if desired, the test object 17 may be tested withplural and mutually different colors of tracer fluid. In thisembodiment, plural vapor cloud generators 20 may be deployed inparallel, two being shown in a nonlimiting embodiment. Each of theplural vapor cloud generators 20 may be supplied from a common airsource 16, such as a plant air line 32.

While a plurality of vapor cloud generators 20 each having a dedicatedexhaust 24 is shown, the invention is not so limited. If desired, pluralvapor cloud generators 20 may feed a single exhaust 24 to provide moreflow volume than achievable with a single vapor cloud generator 20.

In this embodiment, as well as the other embodiments disclosed herein, athrottle valve 31 may be disposed between the air source 16 and theplurality of vapor cloud generators 20. The throttle valve 31 may beused to regulate the volume and pressure of the air flow into the vaporcloud generator 20. If plural vapor cloud generators 20 are deployed,this arrangement provides for each of the vapor cloud generators 20 tohave the same flow characteristics or mutually different flowcharacteristics. This embodiment provides the benefit of allowing forspecific tailoring of the test to the particular test object 17 andexperiment under consideration.

By way of nonlimiting example, first vapor cloud generator 20 may havethe vapor cloud generating liquid 50 spiked with dye to urge the vaporcloud 51 towards a bright white color 51W, while a second vapor cloudgenerator 20 may have the vapor cloud generating liquid 50 spiked withdye to urge the vapor cloud 51 towards a bright dark grey color 51G.This arrangement provides the benefit that for a test object 17 having acomplex geometry, the differently colored vapor cloud 51 streams can beeasily visually discerned and test results more precisely understood.

Referring to FIG. 6A, if desired, the vapor cloud 51 may be drawn intothe test section 11 of the wind tunnel 10 by a vacuum created due to theBernoulli flow of the air in the wind tunnel 10. This arrangementeliminates the need for a positive pressure air source 16 at the inlet21, conserving energy and providing a safer work environment. In such anembodiment, one or more vapor cloud generators 20 may supply a seededvapor cloud 51 to the test section 11 of the wind tunnel 10 with fourvapor cloud generators 20 being shown in a nonlimiting illustration.

In such an embodiment, the vapor cloud generators 20 need not have airsupplied from a separate air source 16. Instead, the test section 11 ofthe wind tunnel 10 may be at a vacuum. That is, the test section 11 mayhave a pressure p_(o) which is less than atmospheric pressure p_(atm).In such an embodiment the Bernoulli effect of the wind tunnel 10 drawsthe vapor cloud 51 from the vapor cloud generator 20, through theexhaust 24 and into the test section 11. This embodiment provides thebenefit that transport of the vapor cloud 51 occurs by operation of thewind tunnel 10 and does not require a dedicated air source 16.

Referring to FIG. 6B, plural vapor generators 20 may provide unequalflows of the vapor cloud 51 to the test section 11 of the wind tunnel10. For example, vapor generator 20 #1 may have an equal or greater flowrate than vapor generator 20 #2, which in turn has an equal or greaterflow rate than vapor generator 20 #3 which in turn has an equal orgreater flow rate than vapor generator 20 #4, etc. Flow rate may becontrolled by a dedicated throttle valve 31 in each exhaust 24 or by theoutput capacity of each vapor generator 20. Likewise two or more vaporgenerators 20 of this embodiment may have mutually different colors ofvapor clouds 51. This embodiment provides the benefit of flexibility indecoupling volume and visual appearance of vapor clouds 51 for complexwind tunnel 10 tests.

The invention is not limited by the specific description above. It is tobe understood the disclosed ranges are approximate and any upper limitof a range may be paired with any lower limit of that same parameter.One of skill will understand various combinations and permutations ofthe above embodiments are feasible and only limited by the scope of theappended claims.

1. A tracer fluid injection system comprising: A sub-ohm resistiveheated vapor cloud generator for generating vapor cloud when a vaporcloud generating liquid is disposed therein, the sub-ohm vapor cloudgenerator having an inlet for receiving air from an air source, wherebyair entering the inlet may be entrained with a vapor cloud beinggenerated by the sub-ohm vapor cloud generator; and having an exhaustfor controlled evacuation of the vapor cloud from the sub-ohm generatorto a test section of a wind tunnel.
 2. A tracer fluid injection systemaccording to claim 1 wherein the sub-ohm vapor cloud generator has achamber for generating vapor cloud therein and subsequent evacuationthrough the exhaust, the chamber having a volume of 1 mL to 10 mL.
 3. Atracer fluid injection system according to claim 2 wherein the chamberhas a volume of 2 mL to 6 mL.
 4. A tracer fluid injection systemaccording to claim 3 wherein the chamber has a generally cylindricalshape defining a longitudinal axis, the chamber being in fluidcommunication with the inlet and the exhaust, the inlet and the exhaustbeing generally diametrically opposed relative to the longitudinal axis.5. A tracer fluid injection system according to claim 1 wherein theexhaust has a flow area of 30 to 130 square millimeters.
 6. A tracerfluid injection system according to claim 1, wherein the exhaustcomprises flexible tubing, the flexible tubing having a length betweenthe chamber and the test section of 1 meter to 4 meters.
 7. A tracerfluid injection system for seeding a wind tunnel, the system comprising:a vapor cloud generator for generating a vapor cloud from a vapor cloudgenerating liquid, the vapor cloud generator comprising a chamber, thechamber having an air flow region and a reservoir in fluid communicationtherewith, the reservoir being in operable relationship with anelectrically powered sub-ohm heating coil for vaporizing a vapor cloudgenerating liquid, containable within the reservoir into tracer fluid, awick for transferring the liquid from the reservoir to the air flowregion, the air flow region having an inlet for receiving air from anair source, whereby air entering the inlet can be entrained with thevapor cloud generated by the sub-ohm vapor cloud coil, the air flowregion having an exhaust in fluid communication with the air flow regionof the chamber and in fluid communication a test section of a windtunnel for controlled evacuation of the vapor cloud from the sub-ohmgenerator to a predetermined position of the test section.
 8. A tracerfluid injection system according to claim 7 wherein the vapor cloudgenerator is disposed within a housing, and the sub-ohm heating coil ispowered by a battery, the battery being disposed in the same housing. 9.A tracer fluid injection system according to claim 7 wherein the inletcomprises an annular ring having an inner perimeter and an outerperimeter opposed thereto, the inner perimeter having an openingcircumscribing the vapor cloud generator for injecting air from theannular ring into the air flow region of the chamber.
 10. A tracer fluidinjection system according to claim 7 further comprising a controller incommunication with a power supply, the sub-ohm coil heating the vaporcloud generating liquid in response to energy from the power supply, thecontroller being programmable to vary the amount of power, duration ofpower and pulse width of power supplied to the from the power supply tothe coil.
 11. A tracer fluid injection system according to claim 10wherein the controller is remotely disposed from the test section of thewind tunnel, whereby an operator can operate the controller and not bein the same room as the wind tunnel during operation.
 12. A tracer fluidinjection system according to claim 11 wherein the controller furtherhas the capability to modulate air flow from the air source to the inletwith respect to at least one of air pressure, air mass flow rate or airspeed.
 13. A tracer fluid injection system according to claim 10comprising the first vapor cloud generator for generating a first vaporcloud and further comprising a second vapor cloud generator forgenerating a second vapor cloud when a second vapor cloud generatingliquid is disposed therein, the second vapor cloud generator having aninlet for receiving air from an air source, whereby air entering theinlet may be entrained with vapor cloud being generated by the secondvapor cloud generator; and having an exhaust for controlled evacuationof the second vapor cloud from the second vapor cloud generator to thetest section of the wind tunnel.
 14. A tracer fluid injection systemaccording to claim 13 wherein the first vapor cloud generator and thesecond vapor cloud generator are charged with first and secondrespective liquids, the first and second respective liquids generatingmutually different colors of vapor clouds during operation.
 15. A tracerfluid injection system according to claim 14 wherein the first vaporcloud exhaust and the second vapor cloud exhaust are directed tomutually different positions of the test section.
 16. A tracer fluidinjection system according to claim 15 wherein the first vapor cloudgenerator and the second vapor cloud generator have mutually mass flowrates during operation and are directed to mutually different positionsof a test object disposed within the test section.
 17. A tracer fluidinjection system according to claim 13 wherein during operation the airsource is at a pressure greater than the pressure of the chamber.
 18. Atracer fluid injection system according to claim 13 wherein the airsource is at ambient pressure the wind tunnel configured to be at apressure less than atmospheric during operation, whereby air can bedrawn from the ambient air source to the inlet of the chamber, throughthe air flow region of the chamber to the exhaust and to the windtunnel.
 19. A method of testing a test object in a wind tunnel, themethod comprising: providing an air source in fluid communication withan inlet of a vapor cloud generator having a chamber, the chamber havinga sub-ohm resistive heating element to vaporize fluid from a reservoir;delivering air from the air source to the inlet of the vapor cloudchamber; applying less than 200 W of power to the resistor whereby theresistive heating element vaporizes a vapor cloud generating liquiddisposed in a reservoir into a vapor cloud, the reservoir being in fluidcommunication with the inlet and in fluid communication with an exhaust;evacuating the vapor cloud from the chamber to the exhaust; directingthe vapor cloud through the exhaust to an operating wind tunnel; theoperating wind tunnel having a test section with a test object therein,the test object being exposed to wind in the test section; and injectingthe vapor cloud from the exhaust to a predetermined position juxtaposedwith the test section.
 20. A method according to claim 19 wherein thestep of applying power to the resistive heating element applies lessthan 60 W of power.